Department of Plant Pathology

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Browsing Department of Plant Pathology by browse.metadata.advisor "Dann, Elizabeth"

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    Assessing Phytophthora cinnamomi seasonal root colonisation patterns and pathogen response to management practices (phosphonates and rootstock tolerance) in South African avocado orchards
    (Stellenbosch : Stellenbosch University, 2019-04) Jolliffe, Jenna Bryanne; McLeod, Adele; Dann, Elizabeth; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Phytophthora root rot (PRR), caused by Phytophthora cinnamomi (Pc), is a destructive soilborne disease that can cause major economic losses in commercial avocado orchards. Despite this, there is limited information on the pathogen’s seasonal colonisation patterns, as well as which sampling strategy and quantification method would be best for assessing it. Current limitations in Pc quantification methods can lead to inaccurate assessments of PRR management strategies including phosphonate fungicides and PRR-tolerant rootstocks. The current study was able to identify a peak in seasonal Pc root colonisation in late autumn (May) in mature avocado orchards situated in two main production regions (Mooketsi and Letaba) in the Limpopo province of South Africa. During two investigated growing seasons (2017 and 2018), Pc root quantities were significantly higher in May 2018 than in March (early autumn), August (late winter) and October/November (late spring) of the same season (2018). In 2017, colonisation trends were less evident, which is likely due to the less conducive PRR conditions that prevailed, especially in the Mooketsi region. In Letaba (2017), August and May yielded the highest Pc root quantities in most orchards, but these did not differ significantly from the other months (March and October/November). In May, Pc root quantities were furthermore significantly positively correlated with the number of hours at soil temperatures of 15-19°C, but negatively with 20-24°C. Soil moisture fluctuations were not associated with Pc root quantities. Evaluation of two sampling strategies consisting of four tree groups (each containing five trees) and one tree group (20 trees), showed that both approaches are suitable for investigating Pc colonisation patterns. A traditional root baiting method, where leaf baits were plated onto selective media, was as effective in identifying colonisation trends as a molecular approach using small-scale root DNA extractions and quantitative real-time PCR (qPCR) analysis. A large-scale root DNA extraction and qPCR analysis method was deemed less effective. A molecular quantification (qPCR) approach was shown to be ineffective for evaluating two management strategies (phosphonate treatments and rootstock tolerance) in avocado orchards showing no obvious aboveground symptoms of PRR decline. Although root phosphite (breakdown product of phosphonates) concentrations of a 2x trunk injection treatment applied at the preventative dosage (0.3 g a.i./m2) were significantly higher than the untreated control, the Pc root and soil DNA concentrations were not significantly reduced by the phosphite, relative to the untreated control. This was for quantifications conducted in either May or October 2018 and using the best of three evaluated Pc-specific qPCR assays. The potentially more PRR-tolerant R0.06 rootstock yielded higher Pc root DNA concentrations than the Dusa® rootstock in November 2017, but not in the other two sampling months (March and May 2018). The identification of effective sampling strategies, Pc root quantification methods and the Pc root colonisation patterns in avocado orchards in the current study is important. Since May had the highest root colonisation levels, PRR management practices should be put in place to achieve optimal root protection during, or just prior to, this period (late autumn). The effective sampling and quantification methods that were identified for studying seasonal root colonisation patterns in avocado, will be useful for other studies that are conducted on the over 5000 host plant species of Pc. Alternative quantification methods to qPCR for assessing management strategies must be investigated. However, it is possible that qPCR analysis may be successful for evaluating management strategies if improvements are made to the trial design, and if analyses are conducted in diseased rather than asymptomatic orchards.